1. Field of the Invention
The present invention relates to a semiconductor laser drive circuit for driving a GaN (gallium nitride)-based semiconductor laser or other high resistance semiconductor laser.
2. Description of the Related Art
Optical disks, types of information recording media, have been made increasingly high in density and large in capacity in recent years.
Along with the increase in density and larger capacity, it is required that the pulse width of emission of laser light be short and that an edge of a waveform of the laser light be sharp.
A semiconductor laser drive circuit of the related art for driving a semiconductor laser controls an output of laser light of the semiconductor laser by switching the size of the drive current.
For example, a semiconductor laser drive circuit of the related art has a plurality of constant current circuits (for plurality of channels) using transistors. It adds the output currents of the constant current circuits and supplies the result the semiconductor laser.
Summarizing the problems of the present invention, with the above semiconductor laser drive circuit of the related art, since there is a parasitic capacity at the output terminal of each constant current circuit, the total parasitic capacity becomes larger along with the increase in number of constant current circuits.
Further, since a constant current circuit is configured by transistors, a mirror effect arises by the capacity between an input and an output.
Due to these factors, the frequency characteristics and transient characteristics deteriorate in a semiconductor laser drive circuit of the related art, thus it becomes difficult to shorten the pulse width of emission of the semiconductor laser and increase the speed of the drive.
Especially, since a GaN-based semiconductor laser for emitting blue-violet laser light has a larger impedance comparing with other semiconductor lasers, it is difficult to drive it at a high speed by a current-control type semiconductor laser drive circuit of the related art.
An object of the present invention is to provide a semiconductor laser drive circuit capable of improving a driving speed of a GaN-based semiconductor laser or other high resistance semiconductor laser.
According to a first aspect of the present invention, there is provided a semiconductor laser drive circuit for driving a high resistance semiconductor laser, comprising a control circuit for generating a drive control signal and a low output impedance drive element for driving the high resistance semiconductor laser by a voltage based on the drive control signal.
Preferably, the control circuit comprises a first switch circuit supplied with a first input signal and a first switch control signal and controlling and outputting a signal level of the first input signal based on the first switch control signal; a second switch circuit supplied with a second input signal and a second switch control signal for controlling and outputting a signal level of the second input signal based on the second switch control signal; and a combining circuit for combining output signals of the first and second switch circuits to generate the drive control signal.
Preferably, the circuit further comprises a constant voltage circuit for generating a biasing use constant voltage signal. Also, the high resistance semiconductor laser is driven in accordance with a plurality of operation modes, and the combining circuit combines the constant voltage signal and an output signal of the first switch circuit when in a first operation mode and combines the constant voltage signal and outputs of the first and second switch circuits when in a second operation mode.
More preferably, the circuit further comprises an oscillation circuit for generating an oscillation signal, wherein the combining circuit combines the constant voltage signal and the oscillation signal when in a third operation mode.
Still more preferably, the circuit further comprises a resonance circuit connected between the oscillation circuit and the combining circuit. The combining circuit combines the oscillation signal passed through the resonance circuit and the constant voltage signal when in the third operation mode.
Preferably, the circuit further comprises a first level-shift circuit supplied with a third input signal for generating the first input signal by shifting the level of the third input signal and a second level-shift circuit supplied with a fourth input signal for generating the second input signal by shifting the level of the fourth input signal.
More preferably, each of the first and second level-shift circuits is a differential amplifying circuit comprised by an emitter coupled logic circuit; the first switch circuit is a current switch logic circuit with a power source connected to an emitter of differential pair transistors and with an output current of the power source set by the first switch control signal; and the second switch circuit is a current switch logic circuit with a power source connected to an emitter of differential pair transistors and with an output current of the power source set by the second switch control signal.
More preferably, the circuit further comprises a first waveform shape circuit supplied with a fifth input signal for shaping a waveform of the fifth input signal to generate the third input signal and a second waveform shape circuit supplied with a fifth input signal for shaping a waveform of the sixth input signal to generate the fourth input signal.
Preferably, the circuit further comprises a power source circuit for supplying a power source voltage to the high resistance semiconductor laser via the low output impedance drive element. The power source circuit outputs the power source voltage to the low output impedance drive element when a control signal of the power source circuit is in an enabled state, while stops outputting the power source voltage when a control signal of the power source circuit is in a disabled state.
Preferably, the circuit further comprises a detection circuit for detecting an inflowing current or an emission intensity of the high resistance semiconductor laser and a feedback circuit for feeding back an output signal of the detection circuit to the low output impedance drive element.
Preferably, the high resistance semiconductor laser comprises a GaN-based semiconductor laser; the low output impedance drive element comprises a field effect transistor; and the GaN-based semiconductor laser is driven by a source follower using the field effect transistor.
According to a second aspect of the present invention, there is provided a semiconductor laser drive circuit for driving a high resistance semiconductor laser, comprising a control circuit for generating a drive control signal; a low output impedance drive element for driving the high resistance semiconductor laser based on the drive control signal by a voltage; and a constant current circuit connected to a connection point of the low output impedance drive element and the high resistance semiconductor laser.
Preferably, the high resistance semiconductor laser and the low output impedance drive element are connected via a capacitor; the connection point of the capacitor and the low output impedance drive element is grounded via a first resistance element; and the constant current circuit is connected to a connection point of the capacitor and the high resistance semiconductor laser.
Preferably, the high resistence semiconductor laser and the constant current circuit are connected via an inductor or a second resistance element.
Preferably, the high resistance semiconductor laser comprises a GaN-based semiconductor laser; the low output impedance drive element comprises a field effect transistor; and the GaN-based semiconductor laser is driven by a source follower using the field effect transistor.
The above first and second aspects of the present invention drive the high resistance semiconductor laser by a low output impedance drive element, so can effectively supply power to the high resistance semiconductor laser.
Also, by supplying power to the high resistance semiconductor laser via a low output impedance drive element, they can suppress the mirror effect and can improve the driving speed of the high resistance semiconductor laser.
By being configured to be powered by a source follower, the high resistance semiconductor laser can suppress the mirror effect in a field effect transistor and can improve the driving speed a GaN-based semiconductor laser or other high resistance laser.
Also since the semiconductor laser drive circuit according to the second aspect of the present invention is provided with a constant current circuit connected to a connection point of a low output impedance drive element and a high resistance semiconductor laser, it can supply a constant current to the high resistance semiconductor laser and can output laser light corresponding to the constant current under normal conditions.